Winter Contingency

First off, my humblest apologies for not including a Mythic guide for Noble Actual. It was just far too difficult for me, but if anyone has the brass to conquer this monster on Mythic, just me know and I'll find a way to squeeze it into the walkthrough.
Sarcasm aside, welcome to the planet Reach, folks, where the celestial skies are breathtakingly beautiful and the landscape is beyond imagination. There couldn't be a better way to introduce you to the new world than a Falcon ride over lush jungles booming with alien life and a dangerously close ringed planet peaking its head over the horizon.
Winter Contingency is relatively easy in terms of difficulty. About 80% of the skirmishes can be skipped if you choose to do so, and more than that, you have plenty of ammo and a team of Spartans to lend you a helping hand. To top it all off, you're almost always in a position where you can either duck into cover or run away entirely in the event that chaos ensues and things get out of hand.
There is but one real "chokepoint" that doesn't rear its ugly head until the very end of the mission, but more on that later. For now, the mission has been divided into three sub-sections:

NOBLE Team

Rebels Don't Leave Plasma Burns

Skeleton Crew

Once the Falcon lands, you'll officially be ready to embark on your Mythic journey through Reach.

NOBLE Team

Remember to press X to exit the Falcon (or LB if you're using Bumper Jumper). Otherwise with Blind activated, you may be sitting in the passenger seat for a few minutes wondering "WTF?" You can investigate the beacon down the hill and to the left if you wish, but skipping ahead won't count against you and it's faster. Also, if you're quick enough, you may catch sight of a camouflaged Elite pocketing one of the game's many secret Data Pads!
Maneuver down the path, through the first structure, and out the other side. Your teammates will instruct you to investigate the first flashing structure you come across, but again it's a useless objective and serves only to intensify the story, and this guide takes into account that you've probably already been through the campaign at least once.
Instead, sprint through to the next set of buildings. When you enter the first, you'll hear the ominous clanging of activity on the roof. That can't be good...
Move through the mini-courtyard and into the next structure. Things are about to heat up. If you take a moment to glance out the windows, you'll find out quickly that NOBLE Team isn't alone. "It's the damned Covenant!"
Make your way down the stairs and beeline it for the great outdoors. While you certainly could spend the next 15-30 minutes fighting out the next few encounters (or the more likely scenario of hiding behind your teammates when you run out of ammo), I have a slightly simpler and faster approach.

Sprint to the overhang to your immediate right, and once you exit, trace the outer edge of the terrain to avoid your enemies. Don't worry about the team. They'll catch up. Besides, you're a rookie. They'll forgive your cowardly acts. If you have a look at what's going on behind you, you'll notice some familiar faces and a few new ones as well. Grunts are back in their dwarfy state, but they've apparently received a hefty upgrade, now sporting "Ultra" armor that shields them from the usual one-shot goodness we're typically familiar with. And apparently they've brought some new friends with them as well. No, those awkward bird-like things aren't Jackals.
Say hello to the Skirmisher. They look and sound nearly identical to Jackals, but they're about as annoying as Drones. They're quick little bastards that dart around and bounce about like toddlers souped up on Mountain Dew. While you really won't need to worry too much about them during the course of this mission, for future reference keep in mind that the further you are away from them, the less dodging they will do.
Moving on, you'll be able to take advantage of the cliff side for the next three mini-battles while your subpar AI buddies try to keep up. The third encounter will consist of three Elite Ultras sporting bright white armor and led by an ambitious General decorated in gold. The former will be using underpowered plasma repeaters and the latter will be taking aim with your greatest threat... the concussion rifle.
At this point in the game you are nowhere near well enough equipped to deal with such adversaries, and waiting for your apparently LD Spartan companions will only result in a massive waste of 10 minutes with nothing to gain but a few extra points for your score. My advice? Just move along and live to fight another day.

Rebels Don't Leave Plasma Burns

Let's hope not anyway.
You'll be presented with your first drivable vehicle I've dubbed the "budget hog". Think Warthog with no rear gun and virtually no armor. Thanks a lot, UNSC! I guess the economy in 2552 sucks too...
Again, you should have it relatively easy here. As shown in the following video, you have the option of avoiding all but the last battle in which the landing zone must be cleared in order for you to move forward.

You can choose to leave your Spartan companions behind if you wish. It won't make a real difference one way or another. Besides, you probably don't want them catching you ruthlessly plowing through herds of fleeing fuzzy ostrich creatures as you speed towards your objective. Admit it. You know you want to.
Follow the path down the hill and teach those Marksman Skirmishers that shooting pink needle things just isn't cool by pancaking them with your stripped down ride. You'll encounter a structure overrun by Covenant troops, but again you really have no reason to stop here. Keep going towards the LZ, and you may be fortunate enough to spot your first real BOB in the form of a golden Ranger Elite. Avoid this guy. He's packing concussion fury and doesn't mess around.
With playtime at last over, it's time to get down to business. You're finally about to kill some enemies!
At the base of the hill, a squad of Marines will already be fighting off a relatively weak team of Covenant. Before the first dropship arrives with reinforcements, I highly recommend using a grenade to launch the downed Warthog off the road to allow easier driving access. Clear roads will highly benefit you during this next skirmish and you're about to find out why.
For the next few minutes, you'll be driving the jeep back and forth across the combat zone splattering enemies to your heart's content. Generally speaking, the Covenant will be too occupied with the Marines and other Spartans to pay much attention to you, but the two big things to look out for here are plasma EMP (yes, Grunts can do this too now), and stray plasma grenades. For the most part, you'll be moving too fast for them to hit you, but just a fair heads up. Go nuts here. Even if for whatever reason you lose the machine, there's another one lying in wait nearby, and besides, on Mythic there's rarely an opportunity to land this many multi-kills in a single battle.
You may also notice the arrival of an old friend here, the Spirit dropship from Halo: Combat Evolved. I've always wondered why they used these when the Phantoms are far more efficient in almost every single way. Perhaps it's a situational thing? Who knows... I do advise, however, that each time a dropship is on the approach, you exit the vehicle and seek cover until it vacates. The concussion bursts it uses can be devastating to both shields and health.
Your main enemies here will consist of Grunt Ultras led by a few Elite Ultras. While you could stick it out and fight them the conventional way, you'll more than likely run out of ammo, and once you do, you'll be forced to let NOBLE team take over. Personally, I like doing the dirty work myself, and this is one of those scenarios where vehicles just seem to be far more efficient at getting the job done.
Once you've managed to clear out the landing zone, the Falcon will swoop down and collect both you and the rest of your team to fly you off to your next objective. Enjoy the ride. It's about to get kar-azy up in here!
What kind of team uses the Rookie as a meat shield on his first day?! You'll notice right away that the pilot sets the bird down with you facing the aggressive Covenant made up of Grunt Ultras and Jackal Majors (identified by their red energy shields). Way to go, NOBLE. Fortunately for you, the opposition is rather light at the moment, but the lull won't last long, so hightail it up to the catwalk to collect a DMR (The battle rifle's single-shot cousin), skirt around the Covenant, and slip inside where you've got plenty of cover and ammunition. From here, you can duck in and out while taking potshots at the cannon fodder.

Once their numbers have been reduced, Spirits will begin descending from the sky and dropping off reinforcements. These troops will include Elite Ultras, so your best bet is to stay in cover and let your team fight this one out as you wait for Kat to activate the blast door. Your biggest threats here are stray plasma grenades and indirect concussion rounds. Neither has to hit you directly in order to put an end your run. After a few short minutes, she'll finally get it right, and the impenetrable doors will start to slide close.
After a few seconds, the door in back will open, and the team will kindly ask you to investigate. Once again, shouldn't the rookie be doing the shadowing here? Nevertheless, do as they ask and proceed forward - only to be taken off guard by three Zealot-class Elites. Good call, NOBLE...

Skeleton Crew

Now the real fun is about to begin. There won't be any skipping from here on out. Just you, your gun, and Hor-hay, though personally, I find Jorge to be the most useful Spartan ally of the bunch.
Your goal at this point is to push back the advancing Covenant composed of Grunts, Jackals, and one Zealot. You'll want to stay in cover as much as possible since the Zealot has a concussion rifle and is damn near ruthless with it. Collect a DMR from the rack on the wall to your right, and rush across the combat zone into cover on the other side. From this angle, you'll be able to advance with very little opposition, but keep your eyes in front for Grunts. It doesn't take much plasma to bring down your shields, and a lucky stick is the last thing you need this late in the mission. And if you're having trouble seeing your enemies, be sure to switch on your night vision, which is basically ODST VISR mode without the enemy/friendly highlights.
For the Grunts, it can take anywhere from 1-3 shots to remove the Ultra mask and one addition shot to finish them off. For the Jackals, I recommend either aiming for the little notch in their energy shields OR their feet. Both will cause them to stumble giving you the chance to land that headshot.
Advancing far enough along the left will cause Jorge to do the same down the center. End result? The Covenant will start retreating into the back corridors. Toss some frags into the bunch to burn through their ranks and finish them off with the DMR. When you proceed forward, do it cautiously. Usually a few Grunts will hang back around the corner and try to blindside you.

After clearing the hall, you have one major area left to take, a dark, cavernous room dimly lit by scattered luminous control panels. The first enemies you'll face here are a group of specialist Grunts. It's rare that you'll come across one-shots in the campaign, so enjoy it while you can. There's a fair amount of distance between you and them, so feel free to take your time and pop them out of existence one by one.
Once you've silenced the cannon fodder, move up the right side carefully. The only things standing between you and victory at this point are two Zealots. This is usually where things go south if handled recklessly. The first of the two will be wielding an energy sword and moves unnaturally fast. Taking him down directly isn't an option at this point. He moves far too fast, and there just isn't enough space to keep him at a safe distance. Lure him back to Jorge, and let the big man do most of the work by engaging him in a melee fight. When the Elite's occupied, sneak up from behind and land a juicy assassination.

One down. One to go!
The final Zealot has a concussion rifle. Since he can shoot from a distance, he'll opt not to follow you like his buddy. Unlike NOBLE team, the Covenant seems to learn from its mistakes. You'll want to use the interior structures to your advantage here to catch him at an angle to where you can peg him with the plasma pistol until he falls over dead (or land that final DMR shot if you have the ammo available). Believe it or not, what the plasma pistol lacks in overcharge capacity, it more than makes up for in overall damage dealt as shown in the next video.

Thanks to your efforts, Kat can now finish her technical work so NOBLE team can continue to fight for Reach. Take a moment to enjoy the rather lengthy cut-scene and bask in the glory of completing for first Mythic mission for Halo: Reach!